Amplifier with active post-distortion linearization

ABSTRACT

An amplifier, which has good linearity and noise performance, includes first, second, third, and fourth transistors and an inductor. The first and second transistors are coupled as a first cascode pair, and the third and fourth transistors are coupled as a second cascode pair. The third transistor has its gate coupled to the source of the second transistor, and the fourth transistor has its drain coupled to the drain of the second transistor. The first transistor provides signal amplification. The second transistor provides load isolation and generates an intermediate signal for the third transistor. The third transistor generates distortion components used to cancel third order distortion component generated by the first transistor. The inductor provides source degeneration for the first transistor and improves distortion cancellation. The sizes of the second and third transistors are selected to reduce gain loss and achieve good linearity for the amplifier.

This application claims the benefit of provisional U.S. Application Ser.No. 60/705,256, entitled “Linearity improvement technique for CMOSamplifiers from low frequency to high frequency by using vectorizedpost-distortion,” filed Aug. 2, 2005.

BACKGROUND

I. Field

The present disclosure relates generally to circuits, and morespecifically to an amplifier suitable for wireless communication andother applications.

II. Background

Amplifiers are commonly used in various electronics devices to providesignal amplification. Furthermore, different types of amplifiers areavailable for different uses. For example, a wireless device may includea transmitter and a receiver for bi-directional communication, and thetransmitter may utilize a power amplifier (PA) and the receiver mayutilize a low noise amplifier (LNA) and a variable gain amplifier (VGA).

An LNA is commonly used in a receiver to amplify a low-amplitude signalreceived via a communication channel. The LNA is often the first activecircuit encountered by the received signal and hence has a large impacton the performance of the receiver in several key areas. First, the LNAhas a large influence on the overall noise figure of the receiver sincethe noise of the LNA is injected directly into the received signal andthe noise of subsequent stages is effectively reduced by the gain of theLNA. Second, the linearity of the LNA has a large influence on both thedesign of subsequent stages in the receiver and the receiverperformance. The LNA input signal typically includes various undesiredsignal components that may come from external interfering sources andleakage from a co-located transmitter. Nonlinearity in the LNA causesthe undesired signal components to mix and generate cross modulationdistortion (XMD) that may fall within the desired signal bandwidth. Theamplitude of the cross cross modulation distortion component that fallswithin the desired signal bandwidth acts as noise that degrades thesignal-to-noise ratio (SNR) of the desired signal. The degradation inSNR caused by LNA nonlinearity impacts the design of (and often placesmore stringent requirements on) subsequent stages in order to meet theoverall SNR specification for the receiver. Therefore, having a morelinear LNA can alleviate the performance requirements for other stages,which may result in lower power consumption and smaller circuit area forthe receiver.

There is therefore a need in the art for an amplifier having goodlinearity and noise performance.

SUMMARY

Various embodiments of an amplifier linearized using activepost-distortion (APD) are described herein. The amplifier is simple indesign, has good linearity and noise performance, and is suitable forwireless communication and other high frequency applications. Forexample, the amplifier may be used as an LNA for a receiver in awireless device. Active post-distortion may also be used to linearizeother active circuits such as, e.g., a mixer.

In an embodiment, an amplifier (e.g., an LNA) includes first, second,third, and fourth transistors (e.g., N-FETs) and an inductor. The firstand second transistors are coupled as a first cascode pair, and thethird and fourth transistors are coupled as a second cascode pair. Thefirst transistor has its source coupled to the inductor and its gatereceiving an input (voltage) signal. The second transistor has itssource coupled to the drain of the first transistor and its drainproviding an output (current) signal. The third transistor has its gatecoupled to the source of the second transistor. The fourth transistorhas its source coupled to the drain of the third transistor and itsdrain coupled to the drain of the second transistor. The firsttransistor provides signal amplification. The second transistor providesload isolation and further generates an intermediate signal for thethird transistor. The third transistor receives the intermediate signaland generates distortion components used to cancel third orderdistortion component generated by the first transistor. The fourthtransistor provides load isolation. The inductor provides sourcedegeneration for the first transistor and improves the cancellation ofthe third order distortion. In other embodiments, the fourth transistormay be omitted, and the drain of the third transistor may be coupled tothe drain of either the first or second transistor. The sizes of thesecond and third transistors may be selected to reduce gain loss for theamplifier and to cancel as much third order distortion as possible.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout.

FIG. 1 shows a radio frequency (RF) portion of a wireless device.

FIGS. 2A, 2B and 2C show a received signal from an antenna, an LNA inputsignal, and an LNA output signal, respectively.

FIG. 3 shows a schematic diagram of an LNA with active post-distortionlinearization.

FIGS. 4A and 4B show plots of IIP3 for the LNA for low and highfrequencies, respectively.

FIG. 5 shows an equivalent circuit for the LNA.

FIG. 6 shows a vector diagram illustrating active post-distortioncancellation.

FIGS. 7A and 7B show schematic diagrams of two additional embodiments ofan LNA with active post-distortion linearization.

FIG. 8 shows a schematic diagram of an LNA with active post-distortionlinearization and multiple gain settings.

FIG. 9 shows a schematic diagram of an LNA implemented with P-FETs.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

The amplifier and other linearized active circuits described herein maybe used for various applications such as communication, networking,computing, consumer electronics, and so on. These linearized activecircuits may be used in wireless communication systems such as a CodeDivision Multiple Access (CDMA) system, a Time Division Multiple Access(TDMA) system, a Global System for Mobile Communications (GSM) system,an Advanced Mobile Phone System (AMPS) system, Global Positioning System(GPS), a multiple-input multiple-output (MIMO) system, an orthogonalfrequency division multiplexing (OFDM) system, an orthogonal frequencydivision multiple access (OFDMA) system, a single-carrier FDMA (SC-FDMA)system, a wireless local area network (WLAN), and so on. The amplifiermay be used as an LNA, a VGA, a PA, and so on. For clarity, an LNA usedin a receiver of a wireless device for a CDMA system is described below.The CDMA system may implement cdma2000, Wideband CDMA (W-CDMA), and/orother CDMA radio access technologies.

FIG. 1 shows a block diagram of a radio frequency (RF) portion of awireless device 100. Wireless device 100 may be a cellular phone, apersonal digital assistant (PDA), a wireless modem card, or some otherdevice used for wireless communication. Wireless device 100 includes atransmitter and a receiver that provide bi-directional communication.

On the transmit path, a power amplifier (PA) 110 receives and amplifiesa transmit (TX) modulated signal and provides a transmit signal. Thetransmit signal is routed through a duplexer 120 and transmitted via anantenna 130 to one or more serving base stations. A portion of thetransmit signal also couples or leaks through duplexer 120 to thereceive path. The amount of TX leakage is dependent on the isolationbetween the transmit and receive ports of duplexer 120, which may beapproximately 50 decibels (dB) for a surface acoustic wave (SAW)duplexer at cellular band. A lower TX-RX isolation results in higherlevel of TX leakage.

On the receive path, a received signal containing a desired signal andpossibly a jammer is received via antenna 130, routed through duplexer120, and provided to an LNA 140. LNA 140 also receives a TX leakagesignal from the transmit path. The input signal at the input of LNA 140may thus include the desired signal, the TX leakage signal, and thejammer. LNA amplifies the input signal and provides an amplified RFsignal. A SAW filter 150 filters the amplified RF signal to removeout-of-band components (e.g., the TX leakage signal) and provides afiltered RF signal. A mixer 160 frequency downconverts the filtered RFsignal with a local oscillator (LO) signal and provides a downconvertedsignal.

FIG. 2A shows the received signal from antenna 130, which includes adesired signal 210 and a jammer 220. Jammer 220 is an undesired signaland may correspond to, for example, a signal transmitted by a nearbybase station in an AMPS system. The jammer may be much higher inamplitude than the desired signal and may be located close in frequencyto the desired signal.

FIG. 2B shows the input signal at the input of LNA 140. The input signalcontains desired signal 210 and jammer 220 in the received signal aswell as a TX leakage signal 230 from the transmit path. The TX leakagesignal may be large relative to the desired signal, especially ifwireless device 100 is far from the serving base station(s) and needs totransmit at a high power level in order to reach the base station(s).

FIG. 2C shows the signal at the output of LNA 140. Nonlinearity in LNA140 can cause the modulation on TX leakage signal 230 to interact withnarrowband jammer 220 and generate cross modulation distortion 240around the jammer. A portion 250 of the cross modulation distortion,which is shown with shading, may fall within the desired signal band.Portion 250 acts as additional noise that degrades the performance ofthe receiver. This noise also degrades receiver sensitivity so that thesmallest desired signal that can be reliably detected by the receiverneeds to have a larger amplitude.

FIG. 3 shows a schematic diagram of an embodiment of an LNA 140 a withactive post-distortion (APD) linearization. LNA 140 a has good linearityand noise performance and may be used for LNA 140 in FIG. 1. LNA 140 aincludes four N-channel field effect transistors (N-FETs) 310, 320, 330and 340, an inductor 350, and a capacitor 352. N-FET 310 has its sourcecoupled to one end of inductor 350, its gate receiving an input voltagev₁, and its drain coupled to the source of N-FET 320. The other end ofinductor 350 couples to circuit ground. N-FET 320 has its gate receivinga bias voltage v_(bias) and its drain coupled to an output node. N-FET330 has its source coupled to circuit ground, its gate coupled to oneend of capacitor 352, and its drain coupled to the source of N-FET 340.The other end of capacitor 352 couples to the source of N-FET 320. N-FET340 has its gate receiving the bias voltage v_(bias) and its draincoupled to the output node. The output node provides an output currenti_(out) for LNA 140 a.

N-FETs 310 and 320 form a first cascode pair for a main signal path usedfor signal amplification. N-FET 310 provides signal amplification. N-FET320 provides load isolation for N-FET 310 and further generates anintermediate voltage v₂ for N-FET 330. N-FETs 330 and 340 form a secondcascode pair for an auxiliary signal path that generates crossmodulation distortion used for distortion cancellation. N-FET 330generates the cross modulation distortion, and N-FET 340 provides loadisolation for N-FET 330. Inductor 350 provides source degeneration andfurther provides a 50-ohm match looking into the gate of N-FET 310.Inductor 350 is also used for active post-distortion linearization andimproves distortion cancellation. Capacitor 352 provides AC coupling.

N-FET 310 has a small-signal transconductance of g₁, which is determinedby various factors such as the size (e.g., length and width) of N-FET310, the bias current for N-FET 310, the gate-to-source voltage v_(gs)of N-FET 310, and so on. N-FET 320 has a small-signal transconductanceof g₁/α, where α is the ratio of the transconductance of N-FET 310 tothe transconductance of N-FET 320. The factor α is typically determinedby the ratio of the width of N-FET 310 to the width of N-FET 320. N-FET330 has a small-signal transconductance of g₁/β, where β is the ratio ofthe transconductance of N-FET 310 to the transconductance of N-FET 330.The factor β is typically determined by the ratio of the width of N-FET310 to the width of N-FET 330. The factors α and β may be selected asdescribed below.

Linearization of LNA 140 a using active post-distortion may be achievedat low frequency as follows. At low frequency, inductor 350 does notcome into play and is effectively shorted, and the input voltage v₁ isequal to the v_(gs) voltage for N-FET 310. The drain current i₁ of N-FET310 may be represented by a power series as:i ₁(v _(gs))=g ₁ ·v _(gs) +g ₂ ·v _(gs) ² +g ₃ ·v _(gs) ³+ . . . ,  Eq(1)where g₂ is a coefficient that defines the strength of second ordernonlinearity;

g₃ is a coefficient that defines the strength of third ordernonlinearity; and

i₁(v_(gs)) is the drain current of N-FET 310 as a function of v_(gs).

For simplicity, nonlinearities higher than third order are ignored inequation (1). Coefficients g₁, g₂ and g₃ are determined by the devicesize and the bias current for N-FET 310. Coefficient g₃ controls thethird order intermodulation distortion (IMD3) at low signal level andhence determines the third order input intercept point (IIP3), which isa metric commonly used to specify the linearity of an amplifier.

N-FET 320 may be assumed to be linear. In this case, the drain voltagev₂ of N-FET 310, which is also the v_(gs) voltage for N-FET 330, may beexpressed as: $\begin{matrix}{v_{2} = {{- \frac{\alpha}{g_{1}}} \cdot {i_{1}.}}} & {{Eq}\quad(2)}\end{matrix}$Equation (2) indicates that the v₂ voltage generated by N-FET 320 isdependent on α. The drain current i₃ of N-FET 330 may be represented bya power series as: $\begin{matrix}{{i_{3}( v_{2} )} = {\frac{1}{\beta}{( {{g_{1} \cdot v_{2}} + {g_{2} \cdot v_{2}^{2}} + {g_{3} \cdot v_{2}^{3}} + \cdots} ).}}} & {{Eq}\quad(3)}\end{matrix}$Equation (3) indicates that the coefficients for N-FET 330 and thecoefficients for N-FET 310 are related by β.

Equation (2) may be substituted into equation (3) so that the draincurrent i₃ of N-FET 330 can be expressed as a function of the draincurrent i₁ of N-FET 310. Equation (1) may then be substituted intoequation (3) so that the drain current i₃ of N-FET 330 can be expressedas a function of the v_(gs) voltage of N-FET 310. The expanded equation(3) includes multiple terms for each order of nonlinearity due to theinteraction between the power series in equation (1) and the powerseries in equation (3).

The drain currents of N-FETs 310 and 330 are combined to generate theoutput current i_(out), as follows: $\begin{matrix}\begin{matrix}{{i_{out} = {i_{1} + i_{3}}},} \\{{= {{g_{1\sum} \cdot v_{gs}} + {g_{2\sum} \cdot v_{gs}^{2}} + {g_{3\sum} \cdot v_{gs}^{3}} + \ldots}},}\end{matrix} & {{Eq}\quad(4)}\end{matrix}$where g_(1Σ) and g_(3Σ) are the first and third order power seriescoefficients, respectively, for the output current i_(out) and may beexpressed as: $\begin{matrix}{{g_{1\sum} = {g_{1} \cdot ( {1 - \frac{\alpha}{\beta}} )}},{and}} & {{Eq}\quad(5)} \\{g_{3\sum} = {{g_{3} \cdot ( {1 - \frac{\alpha}{\beta} - \frac{\alpha^{3}}{\beta}} )} + {\frac{2{g_{2}^{2} \cdot \alpha^{2}}}{g_{1} \cdot \beta}.}}} & {{Eq}\quad(6)}\end{matrix}$The term g_(2Σ) in equation (4) may be ignored since only thefundamental frequency and the third order nonlinearity are of interest.

Equation (5) represents an overall gain for LNA 140 a and shows a gainloss resulting from the use of active post-distortion linearization. Theoverall gain for LNA 140 a with distortion cancellation is g_(1Σ)whereas the gain for the LNA without distortion cancellation is g₁. Thegain loss of (1−α/β) is directly related to α and β and may be keptsmall by selecting β to be large relative to α. A larger β leads to lessgain loss but does not necessarily mean less distortion cancellation.Equation (6) represents the combined third order distortion in theoutput current i_(out). The first term in equation (6) represents thecontribution from third order nonlinearity, and the second term inequation (6) represents the contribution from second order nonlinearity.

FIG. 4A shows a plot 410 of IIP3 for LNA 140 a with distortioncancellation (with N-FETs 330 and 340 connected) and a plot 420 of IIP3for LNA 140 a without distortion cancellation (with N-FETs 330 and 340omitted) at low frequency. For a given device width and powerconsumption, equation (6) may be solved such that the third orderdistortion component approaches zero. The value of β is selected toprevent excessive gain loss. For a specific exemplary design, β isselected to be equal to 8, and a value of 1.35 for α provides gooddistortion cancellation. Because of the second order nonlinearity inequation (6), the distortion cancellation is dependent on bias voltage,which is the operating v_(gs) voltage for N-FET 310.

LNA 140 a may be used for high frequency applications such as wirelesscommunication. At high frequency, reactive elements such as capacitorsand inductors affect linearity performance and further cause performanceto be frequency dependent.

FIG. 5 shows a schematic diagram of a simplified equivalent circuit 500for LNA 140 a in FIG. 3. For the embodiment shown in FIG. 5, N-FETs 310,320, 330 and 340 are modeled with ideal current sources 510, 520, 530and 540, respectively, and parasitic gate-to-source capacitors 512, 522,532 and 542, respectively. N-FETs 310, 320, 330 and 340 havegate-to-source capacitances of C_(gs1), C_(gs2), C_(gs3) and C_(gs4),respectively, and further have gate-to-source voltages of v_(gs1),v_(gs2), v_(gs3) and v_(gs4), respectively. Inductor 350 is modeled withan ideal inductor 550. A circuit 508 models the input impedance Z₁ ofN-FET 310.

For simplicity, the following assumptions are made for equivalentcircuit 500:

All parasitic capacitances are negligible except for C_(gs) for eachN-FET;

Parasitic resistances are zero;

The body effects of the N-FETs are negligible; and

LNA 140 a operates in a weakly nonlinear region with a small inputsignal v₁.

The drain currents for current sources 510, 520 and 530 may be expressedas: $\begin{matrix}{{i_{{ds}\quad 1} = {{g_{1} \cdot v_{{gs}\quad 1}} + {g_{2} \cdot v_{{gs}\quad 1}^{2}} + {g_{3} \cdot v_{{gs}\quad 1}^{3}}}},} & {{Eq}\quad( {7a} )} \\{{i_{{ds}\quad 2} = {\frac{g_{1}}{\alpha} \cdot v_{{gs}\quad 2}}},{and}} & {{Eq}\quad( {7b} )} \\{{i_{{ds}\quad 3} = {\frac{1}{\beta} \cdot ( {{g_{1} \cdot v_{{gs}\quad 3}} + {{g_{2} \cdot {v_{{gs}\quad 3}^{2}++}}{g_{3} \cdot v_{{gs}\quad 3}^{3}}}} )}},} & {{Eq}\quad( {7c} )}\end{matrix}$where v₂=v_(gs3)=−v_(gs2). For simplicity, only the nonlinearities ofN-FETs 310 and 330 are considered, and N-FETs 320 and 340 are assumed tobe linear, as indicated by equation (7b).

The output current i_(out) for equivalent circuit 500 in a weaklynonlinear region may be expressed as:i _(out) =C ₁(s)∘v ₁ +C ₂(s ₁ ,s ₂)∘v ₁ ² +C ₃(s ₁ ,s ₂ ,s ₃)∘v ₁ ³,  Eq(8)

-   where C_(n)(s₁, . . . ,s_(n)) is a Laplace transform of the n-th    order Volterra kernel for i_(out), which is often called the n-th    order nonlinear function;    -   s=jω is the Laplace variable;    -   s₁, . . . , s_(n) are frequencies operated on by the n-th order        Volterra kernel; and    -   “∘” denotes a complex multiply of each frequency component of v₁        ^(n) by C_(n)(s₁, . . . ,s_(n)).

Equation (8) is for a Volterra series that is often used for nonlinearanalysis. The Volterra series includes a Volterra kernel for each orderof nonlinearity. The n-th order nonlinearity corresponds to the term v₁^(n) and generates n frequency components. The n-th Volterra kernel is aset of n coefficients that operates on the n frequency componentsgenerated by the n-th order nonlinearity. The coefficients for eachVolterra kernel may be determined by mathematical derivation or someother means. In equation (8), the third order Volterra kernelC₃(s₁,s₂,s₃) determines third order nonlinearity at high frequency,which is of interest.

The gate-to-source voltage v_(gs1) of N-FET 310 may be expressed as afunction of the input voltage v₁, as follows:v _(gs1) =A ₁(s)∘v ₁ +A ₂(s ₁ ,s ₂)∘v ₁ ² +A ₃(s ₁ ,s ₂ ,s ₃)∘v ₁ ³,  Eq(9)where A_(n)(s₁, . . . , s_(n)) is the Laplace transform of the n-thorder Volterra kernel for v_(gs1).

N-FET 310 generates a nonlinear current i_(ds1) based on the inputvoltage v₁, as shown in equations (7a) and (9). A portion of the ids,current passes through N-FET 320 and generates the v₂ voltage. The v₂voltage generates a nonlinear current i_(ds3) through N-FET 330, asshown in equation (7c). The output current i_(out) is equal to the sumof the i_(ds1) current and the i_(ds3) current.

Equation (8) may be evaluated to determine all distortion components.The distortion components of interest are those that affect IIP3. Thedistortion components generated by third order nonlinearity of N-FET 310are denoted as Sζ_(M1). The distortion components generated bynonlinearities of N-FET 330 may be categorized as follows:

-   -   ζ₁: distortion components generated by second and third order        nonlinearities of N-FET 310 and attenuated by a factor of α/β;    -   ζ₂: distortion components generated by second order nonlinearity        of N-FET 310 multiplied by second order nonlinearity of N-FET        330; and    -   ζ₃: distortion components generated by third order nonlinearity        of N-FET 330.        With active post-distortion linearization, the terms ζ₁, ζ₂ and        ζ₃ are actively generated with N-FET 330 and are used to cancel        the term ζ_(M1) from N-FET 310.

The term ζ₁ includes distortion components generated by second and thirdorder nonlinearities of N-FET 310. For example, the second harmonic (2ω)at the source of N-FET 310 can mix with the fundamental frequency (ω) atthe gate of N-FET 310 to generate third order intermodulationdistortion. The second harmonic is due to second order nonlinearity ofN-FET 310, which corresponds to the term g₂·v_(gs1) ² in equation (7a).The fundamental frequency can also generate third order intermodulationdistortion due to third order nonlinearity of N-FET 310, whichcorresponds to the term g₃·v_(gs1) ³ in equation (7a). These distortioncomponents from N-FET 310 are amplified by N-FET 330 through theg₁·v_(gs3) term in equation (7c) and are attenuated by a factor of α/βby the combination of N-FETs 320 and 330.

The term ζ₂ includes distortion components generated by second ordernonlinearities of N-FETs 310 and 330. For example, the second harmonicgenerated by the second order nonlinearity of N-FET 310 can mix with thefundamental frequency due to the second order nonlinearity of N-FET 330,which corresponds to the term g₂·v_(gs3) ² in equation (7c), to generatethird order intermodulation distortion.

The term ζ₃ includes distortion components generated by third ordernonlinearity of N-FET 330. The fundamental frequency from N-FET 310 cangenerate third order intermodulation distortion due to third ordernonlinearity of N-FET 330, which corresponds to the term g₃·v_(gs3) ³ inequation (7c).

The nonlinearity terms for N-FETs 310 and 330 may be expressed as:$\begin{matrix}{{\zeta_{M\quad 1} = {{g_{1} \cdot {A_{3}( {s_{1},s_{2},s_{3}} )}} + {2{g_{2} \cdot \overset{\_}{{A_{1}( s_{1} )} \cdot {A_{2}( {s_{1},s_{2}} )}}}} + {g_{3} \cdot {A_{1}( s_{1} )} \cdot {A_{1}( s_{2} )} \cdot {A_{1}( s_{3} )}}}},} & {{Eq}\quad(10)} \\{{\zeta_{1} = {{- \frac{\alpha}{\beta}} \cdot \zeta_{M\quad 1}}},} & {{Eq}\quad(11)} \\{{\zeta_{2} = {{{\frac{g_{2} \cdot \alpha^{2}}{\beta} \cdot \lbrack \quad }2{g_{1}^{2} \cdot \overset{\_}{{A_{1}( s_{1} )} \cdot {A_{2}( {s_{1},s_{2}} )}}}} + {2{g_{1} \cdot g_{2} \cdot {A_{1}( s_{1} )} \cdot {A_{1}( s_{2} )} \cdot {A_{1}( s_{3} )}} \quad \rbrack}}},{and}} & {{Eq}\quad(12)} \\{{\zeta_{3} = {{- \frac{g_{3} \cdot \alpha^{3}}{\beta}} \cdot g_{1}^{3} \cdot {A_{1}( s_{1} )} \cdot {A_{1}( s_{2} )} \cdot {A_{1}( s_{3} )}}},} & {{Eq}\quad(13)} \\{{{{where}\quad{A_{1}(s)}} = \frac{1}{{s \cdot L_{s} \cdot g_{1}} + {s \cdot C_{{gs}\quad 1} \cdot ( {{s \cdot L_{s}} + {Z_{1}(s)}} )} + 1}},} & {{Eq}\quad(14)} \\{{\overset{\_}{{A_{1}( s_{1} )} \cdot {A_{2}( {s_{1},s_{2}} )}} = {{{- \frac{1}{3}} \cdot {A_{1}(s)} \cdot {{A_{1}(s)}}^{2} \cdot {A_{1}( {2s} )} \cdot 2}{s \cdot L_{s} \cdot g_{2}}}},{and}} & {{Eq}\quad(15)} \\{{A_{3}( {s_{1},s_{2},s_{3}} )} = {s \cdot L_{s} \cdot {A_{1}^{2}(s)} \cdot {{A_{1}(s)}}^{2} \cdot {\lbrack {{{\frac{2}{3} \cdot g_{2}^{2} \cdot {A_{1}( {2s} )} \cdot 2}{s \cdot L_{s}}} - g_{3}} \rbrack.}}} & {{Eq}\quad(16)}\end{matrix}$

Equations (14), (15) and (16) indicate that the inductance L_(s) ofinductor 350 is included in various intermediate terms that make up ζ₁,ζ₂ and ζ₃. Inductor 350 improves cancellation of third order distortiongenerated by N-FET 310, at high frequency.

In the above equations, s=jω, s₁=jω₁, s₂=jω₂, and s₃=jω₃ are differentclosely spaced signal frequencies, with ω≈ω₁≈ω₂≈ω₃, so that Δω=ω₂−ω₁ ismuch smaller than ω₁ and ω₂. Substituting equations (14), (15) and (16)into equations (10), (11), (12) and (13), and assuming conjugate matchat ω, the total third order distortion in the output current i_(out),IM3_(Σ), may be expressed as: $\begin{matrix}{{{IM}\quad 3_{\sum}} = {{{A_{1}(s)} \cdot {{A_{1}(s)}}^{2} \cdot ( {1 - \frac{\alpha}{\beta}} ) \cdot g_{3} \cdot \lbrack {\frac{1}{2} - \frac{\alpha^{3}}{\beta - \alpha}} \rbrack} - {{A_{1}(s)} \cdot {{A_{1}(s)}}^{2} \cdot ( {1 - \frac{\alpha}{\beta}} ) \cdot ( {{\frac{2}{3} \cdot g_{2}^{2} \cdot {A_{1}( {2s} )} \cdot 2}{s \cdot L_{s} \cdot \lbrack {\frac{1}{2} + \frac{\alpha^{2}}{\beta - \alpha}} \rbrack}} )} + {2{\frac{\alpha^{2} \cdot g_{2}^{2}}{\beta \cdot g_{1}} \cdot {{A_{1}^{3}(s)}.}}}}} & {{Eq}\quad(17)}\end{matrix}$IM3_(Σ) in equation (17) corresponds to the third order Volterra kernelC₃(s₁,s₂,s₃) in equation (8).

In equation (17), the term in the first row represents third ordernonlinearity, the term in the second row represents second ordernonlinearity with second order harmonic, and the term in the third rowrepresents second order nonlinearity. The values of α and β may beselected such that these three distortion components cancel out as muchas possible, the total third order distortion is minimized, and thehighest possible IIP3 is achieved for LNA 140 a.

FIG. 6 shows a vector diagram that illustrates the distortioncancellation mechanism of active post-distortion. The terms ζ₁, ζ₂ andζ₃ are dependent on the signal frequency (s=jω), the coefficients g₁, g₂and g₃ of the N-FETs, and the degeneration inductance L_(s). The termsζ₁, ζ₂ and ζ₃ can have different amplitudes and phases at a givenfrequency, as shown by the three vectors for these three terms. The sumof the three terms ζ₁, ζ₂ and ζ₃ is shown by a dashed vector, whichshould be equal in amplitude but opposite in phase with the vector forζ_(M1), so that the total distortion is minimize.

FIG. 4B shows a plot 430 of IIP3 for LNA 140 a with distortioncancellation and a plot 440 of IIP3 for LNA 140 a without distortioncancellation at high frequency. For a given device width and powerconsumption, equation (17) may be solved to make the third orderdistortion component approach zero. The value of β is selected toprevent excessive gain loss. For a specific exemplary design, β isselected to be equal to 8, and a value of 1.77 for α provides gooddistortion cancellation. The value for α that minimizes distortion athigh frequency may be different from the value for α at low frequency.The different α for high frequency is due to the distortion componentgenerated by second order nonlinearity interacting with second harmonic,which corresponds to the second line in equation (17).

The noise performance of LNA 140 a is degraded slightly with activepost-distortion linearization. The noise from N-FET 310 is approximatelythe same as the noise from a conventional inductively degenerated LNA.With active post-distortion linearization, additional noise is generatedby N-FET 330 in the form of gate induced noise and drain noise. Both ofthese additional noise sources may be reduced by increasing β, whichresults in less gain loss and smaller degradation in noise figure.

FIG. 7A shows a schematic diagram of an embodiment of an LNA 140 b withactive post-distortion linearization. LNA 140 b includes N-FETs 310, 320and 330, inductor 350, and capacitor 352, which are coupled as describedabove for FIG. 3. However, the drain of N-FET 330 is coupled directly tothe output node. N-FET 340 is omitted in LNA 140 b. The linearity andnoise performance of LNA 140 b is similar to that of LNA 140 a in FIG.3. Omitting N-FET 340 mainly affects load isolation for N-FET 330.

FIG. 7B shows a schematic diagram of an embodiment of an LNA 140 c withactive post-distortion linearization. LNA 140 c includes N-FETs 310, 320and 330, inductor 350, and capacitor 352, which are coupled as describedabove for FIG. 3. However, the drain of N-FET 330 is coupled directly tothe source of N-FET 320. N-FET 340 is omitted in LNA 140 c. Thelinearity and noise performance of LNA 140 c is similar to that of LNA140 a in FIG. 3.

FIG. 8 shows a schematic diagram of an embodiment of an LNA 140 d withactive post-distortion linearization and multiple gain settings. LNA 140d includes N-FETs 810, 820, 830 and 840, an inductor 850, and acapacitor 852 that are coupled in the same manner as N-FETs 310, 320,330 and 340, inductor 350, and capacitor 352, respectively, in FIG. 3.LNA 140 d further includes additional circuitry that provides biasing,gain control, and impedance matching.

The bias circuitry for LNA 140 d includes a current source 858, an N-FET860, and resistors 862, 864, 866 and 868. Current source 858 has one endcoupled to a power supply V_(DD) and the other end coupled to the drainof N-FET 860. N-FET 860 is diode connected and has its source coupled tocircuit ground and its gate coupled to its drain. Resistor 862 has oneend coupled to the gate of N-FET 810 and the other end coupled to thegate of N-FET 860. Resistor 864 has one end coupled to the gate of N-FET830 and the other end coupled to the gate of N-FET 860. The bias currentfor N-FET 810 is determined by (1) the current provided by currentsource 858 and (2) the ratio of the width of N-FET 810 to the width ofN-FET 860. Similarly, the bias current for N-FET 830 is determined by(1) the current provided by current source 858 and (2) the ratio of thewidth of N-FET 830 to the width of N-FET 860. Resistor 866 has one endcoupled to the V_(DD) supply and the other end coupled to the gates ofN-FETs 820 and 840. Resistor 868 has one end coupled to circuit groundand the other end coupled to the gates of N-FETs 820 and 840. Resistors866 and 868 determine the gate bias voltage for N-FETs 820 and 840,which does not need to be precisely set.

The gain control circuitry for LNA 140 d includes N-FETs 870 and 880, acapacitor 872, and resistors 882, 884 and 886. N-FETs 870 and 880 havetheir sources coupled to the gate of N-FET 810 and their gates receivingtwo gain control signals. Capacitor 872 has one end coupled to thedrains of N-FETs 820 and 840 and the other end coupled to the drain ofN-FET 870. Resistors 882 and 884 are coupled in series. Resistor 882 hasone end coupled to the drain of N-FET 880 and the other end coupled toresistors 884 and 886. The other end of resistor 884 is coupled to thedrains of N-FETs 820 and 840, and the other end of resistor 886 iscoupled to circuit ground.

N-FETs 810, 820, 830 and 840 form a gain signal path, N-FET 870 forms apass-through signal path, and N-FET 880 forms an attenuation signalpath. One of the three signal paths is selected as any given momentbased on the two gain control signals. If N-FET 870 is turned on and thepass-through signal path is selected, then the input signal passesthrough N-FET 870 and AC coupling capacitor 872 to the LNA output. IfN-FET 880 is turned on and the attenuation signal path is selected, thenthe input signal passes through N-FET 880 and is attenuated by theresistor network.

An input impedance matching circuit 890 couples between an RF input andthe gate of N-FET 810. An output impedance matching circuit 892 couplesbetween an RF output and the V_(DD) supply. Each impedance matchingcircuit may include one or more inductors, capacitors, strip lines, andso on. Matching circuit 892 also provides bias current for N-FETs 810,820, 830, 840 and 880.

FIG. 9 shows a schematic diagram of an embodiment of an LNA 140 e withactive post-distortion linearization. LNA 140 e includes four P-channelFETs (P-FETs) 910, 920, 930 and 940, an inductor 950, and a capacitor952. P-FET 910 has its source coupled to one end of inductor 950, itsgate receiving the input voltage v₁, and its drain coupled to the sourceof P-FET 920. The other end of inductor 950 couples to the V_(DD)supply. P-FET 920 has its gate receiving the bias voltage v_(bias) andits drain coupled to the output node. N-FET 930 has its source coupledto the V_(DD) supply, its gate coupled to one end of capacitor 952, andits drain coupled to the source of P-FET 940. The other end of capacitor952 couples to the source of P-FET 920. P-FET 940 has its gate receivingthe bias voltage v_(bias) and its drain coupled to the output node. Theoutput node provides the output current i_(out) for LNA 140 e.

As noted above, the techniques for linearizing an active circuit usingactive post-distortion may be used for various types of active circuitsuch as amplifier, mixer, and so on. The main signal path for the activecircuit generates distortion due to nonlinearity of the circuit elementsin the main signal path. An auxiliary signal path actively generatesdistortion components used to cancel the distortion components generatedby the main signal path.

The amplifier and other linearized active circuits described herein maybe used for various frequency ranges including baseband, intermediatefrequency (IF), RF, and so on. For example, these linearized activecircuits may be used for frequency bands commonly employed for wirelesscommunication, such as:

-   -   Cellular band from 824 to 894 MHz,    -   Personal Communication System (PCS) band from 1850 to 1990 MHz,    -   Digital Cellular System (DCS) band from 1710 to 1880 MHz,    -   GSM900 band from 890 to 960 MHz,    -   International Mobile Telecommunications-2000 (IMT-2000) band        from 1920 to 2170 MHz, and    -   Global Positioning System (GPS) band from 1574.4 to 1576.4 MHz.

The amplifier and other linearized active circuits described herein maybe implemented within an integrated circuit (IC), an RF integratedcircuit (RFIC), an application specific integrated circuit (ASIC), aprinted circuit board (PCB), an electronic device, and so on. Theselinearized active circuits may also be fabricated with various ICprocess technologies such as complementary metal oxide semiconductor(CMOS), N-channel MOS (N-MOS), P-channel MOS (P-MOS), bipolar junctiontransistor (BJT), bipolar-CMOS (BiCMOS), silicon germanium (SiGe),gallium arsenide (GaAs), and so on.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An integrated circuit comprising: a first transistor electricallycoupled to an inductor and operative to receive and amplify an inputsignal; a second transistor electrically coupled to the first transistorand operative to generate an intermediate signal and to provide anoutput signal; and a third transistor electrically coupled to the secondtransistor and operative to receive the intermediate signal and generatedistortion components used to cancel a distortion component generated bythe first transistor.
 2. The integrated circuit of claim 1, wherein thefirst and second transistors are coupled as a cascode pair.
 3. Theintegrated circuit of claim 1, wherein the third transistor is operativeto generate distortion components used to cancel third order distortioncomponent generated by the first transistor.
 4. The integrated circuitof claim 1, further comprising: a fourth transistor electrically coupledto the second and third transistors and operative to provide loadisolation.
 5. The integrated circuit of claim 1, wherein the thirdtransistor has a drain electrically coupled to a drain of the secondtransistor.
 6. The integrated circuit of claim 1, wherein the thirdtransistor has a drain electrically coupled to a drain of the firsttransistor.
 7. The integrated circuit of claim 1, wherein the first,second, and third transistors have first, second, and third gains,respectively, wherein the first and second gains are related by a firstfactor, and wherein the first and third gains are related by a secondfactor.
 8. The integrated circuit of claim 7, wherein the second factoris selected to reduce gain loss, and wherein the first factor isselected to cancel the distortion component generated by the firsttransistor.
 9. The integrated circuit of claim 7, wherein the secondfactor is greater than one, and wherein the third gain is a fraction ofthe first gain.
 10. The integrated circuit of claim 1, wherein thefirst, second, and third transistors are N-channel field effecttransistors (N-FETs).
 11. The integrated circuit of claim 1, wherein thefirst, second, and third transistors are P-channel field effecttransistors (P-FETs).
 12. The integrated circuit of claim 1, wherein thefirst, second, and third transistors are bipolar junction transistors(BJTs).
 13. The integrated circuit of claim 1, wherein the first,second, and third transistors form a low noise amplifier (LNA).
 14. Theintegrated circuit of claim 1, further comprising: a gain controlcircuit electrically coupled to the first and second transistors andoperative to provide gain control for an amplifier formed by the first,second, and third transistors.
 15. The integrated circuit of claim 14,wherein the gain control circuit comprises a fourth transistorelectrically coupled to the first transistor; and a capacitorelectrically coupled between the fourth transistor and the secondtransistor.
 16. The integrated circuit of claim 14, wherein the gaincontrol circuit comprises a fourth transistor electrically coupled tothe first transistor; and at least one resistor electrically coupledbetween the fourth transistor and the second transistor.
 17. Theintegrated circuit of claim 1, wherein the input signal is a CodeDivision Multiple Access (CDMA) signal.
 18. An amplifier comprising: aninductor operative to provide source degeneration; a first transistorhaving a source electrically coupled to the inductor and a gatereceiving an input signal, the first transistor operative to providesignal amplification; a second transistor having a drain providing anoutput signal and a source electrically coupled to a drain of the firsttransistor, the second transistor operative to generate an intermediatesignal; and a third transistor having a gate electrically coupled to thesource of the second transistor, the third transistor operative toreceive the intermediate signal and generate distortion components usedto cancel a distortion component generated by the first transistor. 19.The amplifier of claim 18, further comprising: a fourth transistorhaving a source electrically coupled to a drain of the third transistorand a drain electrically coupled to the drain of the second transistor.20. An apparatus comprising: means for amplifying an input signal togenerate a first signal having a desired component and a distortioncomponent; means for generating a second signal having distortioncomponents used to cancel the distortion component generated by theamplifying means; and means for combining the first and second signalsto generate an output signal having the distortion component generatedby the amplifying means canceled.
 21. The apparatus of claim 20, whereinthe distortion component generated by the amplifying means is a thirdorder distortion component.
 22. The apparatus of claim 20, furthercomprising: means for controlling gain of the output signal.
 23. Areceiver for a wireless device, comprising: a low noise amplifier (LNA)comprising an inductor operative to provide source degeneration, a firsttransistor having a source electrically coupled to the inductor andoperative to provide signal amplification, a second transistor having asource electrically coupled to a drain of the first transistor andoperative to generate an intermediate signal, and a third transistorhaving a gate electrically coupled to the source of the secondtransistor, the third transistor operative to receive the intermediatesignal and generate distortion components used to cancel a distortioncomponent generated by the first transistor; an input impedance matchingcircuit electrically coupled to a gate of the first transistor andreceiving an input signal for the LNA; and an output impedance matchingcircuit electrically coupled to a drain of the second transistor andproviding an output signal for the LNA.
 24. The receiver of claim 23,wherein the LNA further comprises a fourth transistor having a sourceelectrically coupled to a drain of the third transistor and a drainelectrically coupled to the drain of the second transistor.
 25. Thereceiver of claim 23, wherein the LNA further comprises a gain controlcircuit electrically coupled to the first and second transistors andoperative to provide gain control for the LNA.